In our discussion of wave energetics (Chap. 4 ), we proved that waves slow down as they enter shallower water while increasing their height, a phenomenon referred to as wave shoaling. This increase in wave height is due to the increase in energy density since the energy flux must remain constant. Once this height reaches a critical value, the crest collapses back into the water in a process known as wave breaking. Understanding the extent to which deep-water waves increase their height as they propagate into coastal regions and the conditions by which breaking occurs is important for the design of many types of coastal structures. This is because when designing such structures to resist wave forces or overtopping, local wave heights at the location of the structure must be employed when carrying out the required calculations.